JPS6355791B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to control of drive current for light emitting semiconductor devices, and more particularly to feedback control of bias current for injection lasers.

Injection lasers are light emitting semiconductor devices that act as efficient and compact coherent light sources. Such devices are used in data communications and recording, printing and display devices.

In many applications, injection lasers must be supplied with a bias current to operate satisfactorily.
Examples of using a fixed bias current for an injection laser are, for example, U.S. Pat.
See specifications No. 3925735, No. 3968399 and No. 4027179.

Feedback control of laser bias current is also known. If the bias current is controlled by a negative feedback system responsive to optical output, differences in individual device characteristics and changes in device characteristics over time due to aging or temperature changes, etc., are compensated for.

Examples of the use of bias current feedback control for injection lasers are found in, for example, U.S. Pat.
“Electronic Circuits for High Bit Rate
Digital Fiber Optic Communication
Systems”, IEEE Trans.Comm., COM−26 ,
1088-1098 (July1978); D. Smith, “Laser Level
−control Ciruit for High−bit−rate System
Using a Slope Detector”, Elect.Lett., 14 ,
775-6 (1978); P. Schumate et al., “GaAlAs
Laser Transmitter for Lightwave
Transmission System”Bell Sys.Tech.Jour. 57 ,
Found in 1823-1836 (1978).

All these feedback systems are of the analog type even when the injection laser modulation signal is digital in nature. This is clearly because the amplitude of the sensed light is not converted into a digital code, nor is a bias current created by converting the digital representation (as opposed to a modulating signal) into analog form. It is from. The bias current is analog-like, as is clear from the fact that the amplitude values it can take are continuous. If the bias current is created as a result of a conversion from a digital code, then the bias current is a discrete value in the steady state (after the transient from one discrete value to another has disappeared). I can only take it. Therefore, the prior art used analog feedback to control the bias level of the injection laser, even though the signal superimposed on the controlled bias level was typically of a digital nature.

Conventional circuits include circuit elements such as capacitors and inductors that are inherently difficult to integrate, so they are inherently difficult to monolithically integrate. These circuit elements are necessary in such analog circuits because the analog circuit requires one or more low pass filters. A low-pass filter can be used, for example, to prevent the modulating signal from negatively impacting the negative feedback bias stabilization system.
and in the feedback system itself to perform operations such as integration. Also, prior art circuits do not provide extended or flexible control or testing capabilities. This is because each additional function requires additional circuitry. As a result, prior art control circuits lacked flexibility. Different modes of operation or different applications usually require different control circuits.

Monolithic integration offers price and performance advantages over other implementations. It also makes the control system small enough to be placed in the same module as the injection laser and the attached output optical fiber. Additionally, expanded control and test capabilities provide application flexibility that is typically associated with digital circuitry.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a bias current control circuit for an injection laser that can be easily monolithically integrated.

Other objects of the invention are extended flexible control;
The object of the present invention is to provide a control circuit with modulation and test functions.

Another object of the invention is to provide such a control circuit that can be manufactured and tested on a normal digital component production line.

Another object of the invention is to provide a control circuit that is small enough to be implemented in the same module as the injection laser and the attached output optical fiber.

Another object of the invention is to provide a digital control circuit for an injection laser.

Drive current for a light emitting semiconductor device, such as an injection laser, is provided by a digitally programmable current source that can provide any one of a plurality of different discrete current amplitudes. The amplitude of light emitted by the device is sensed and digitally encoded. A logic circuit responsive to the digitally encoded light output level and selectively responsive to other digital signals as well controls the discrete current amplitude provided by the programmable current source.

When the preferred embodiment is used for digital (or analog) communications, a digitally programmable current source provides bias current for the injection laser. A modulation current is added to this bias current to form the total drive current for the laser. In another mode of operation, a modulated drive current is provided by a programmable current source.

As shown in FIG. 1, light receiving device 10 is positioned with respect to light emitting semiconductor device 12 so as to be responsive to light 14 emitted by light emitting semiconductor device 12. As shown in FIG. Light 14 from device 12 also travels in the other direction, where it performs one or more useful functions. For example, light 14
is an optical fiber 16 as shown in FIGS. 1 and 2.
May be combined with Devices 10 and 12 may be mounted on common structure 18 in any convenient manner. Digital controller 20 generates a drive current on signal line 24 for device 12 in response to analog signals on signal line 22 coming from device 10 . Digital controller 20 is generally comprised of an analog-to-digital converter 26, control logic 28, and a digital-to-analog converter 30.
Controller 20 is generally configured for digital interconnection with other digital devices, such as digital system 32 of FIG. Digital system 32 includes digital control word line 3
The control information is given to the control device 20 via step 4. Controller 20 also reports status information to digital system 32 via digital status report word line 36. In many applications, light emitting device 12 is modulated in some manner. Modulation information is typically sent from digital system 32 to controller 20 via code input line 38. Modulation information can also be sent to the control device 2 from devices other than the system 32.
given to 0. Also, the modulation information may not be used to control the output, but rather be summed with the controlled output 24. Furthermore, the modulation information may be analog instead of digital.

Although device 12 can be virtually any light emitting semiconductor device, such as an injection laser or an LED, the digital controller has features that are most advantageously used when the device is an injection laser. An example of a control device to be described in detail below is therefore a control device for an injection laser. The control device can be used in other light emitting devices with only minor modifications obvious to those skilled in the art.

FIG. 2 shows a preferred embodiment of the control device. Here, the analog-to-digital converter is implemented as a two-threshold comparator 40, the control logic is implemented as an up/down counter 42, and the digital-to-analog converter is implemented as a digitally programmable current source 44. This controller has several different modes of operation and can be used in many different applications. In the embodiment shown in FIG. 2, the light emitting device is an injection laser 46 and the light receiving device is a photodiode 48. In this application, laser 46 is used to transmit optical signals into optical fiber 16. The input signal to the signal line 50 may be a digital signal of two or more levels or an analog signal. The optical signal generated by laser 46 corresponds to the input signal. For ease of explanation, input signal 50 is shown added to output signal 52 from digitally programmable current source 44 . However, the two signals may also be summed inside the current source 44. The programmable current source also receives an encoded input signal and converts it into an output signal 52.
may be deciphered or converted into the form of its components.

FIG. 3 is a graph of a typical injection laser output power curve. When the drive current exceeds the threshold _ITH , the device starts lasing. Following I _TH there is an almost linear region 58 where the device is fully lased. The slope of region 58 is I _TH
It is steeper than the following areas. Three region definitions are superimposed on the optical power output curve. The laser is controlled so that the average output power remains within the region labeled with the binary code "11". Region "11" is thus spaced well above P _{TH so that expected variations in output power do not intersect P TH .}
On the other hand, region "11" should not be so far above the curve that the lifetime of the laser is unduly shortened. Area “01”
is defined to be the power region below region "11", and region "10" is defined to be the region above region "11".

Two threshold comparators 40 determine whether the laser is operating in region "11" or in region "10" or "01". P ₁ is the lower threshold and corresponds to the drive current I ₁ . The other threshold is defined by P ₂ and corresponds to the drive current I ₂ . Region "11" is defined to be the power region between these two thresholds. Up/down counter 42 is responsive to the determination of comparator 40. When the laser is in the area "11", the counter 42
holds the current count value. Laser area "01"
, the counter 42 gradually changes the count value in a direction in which the laser power tends to increase. When the laser is in the area "10", the counter 42
gradually changes the count value in the opposite direction so that the laser power has a decreasing slope. The count value of counter 42 is converted by a digitally programmable current source 44 into a corresponding bias current for the laser.

FIG. 4 is a more detailed illustration of the embodiment schematically shown in FIG. The logic circuit explained in Fig. 4 is the third
It is designed to generate the code described in the figure. Other structures can also be used to generate the same code. The numbers in FIG. 3 are those used in the commercially available logic chips used to implement portions of this circuit. Other chips requiring other codes may be used, but the structure of the logic circuit must then be modified accordingly. It is assumed that FIG. 4 is implemented using emitter coupling logic.
Other logic techniques could be used, with obvious modifications. photodiode 48
is positioned with respect to laser 46 such that it is illuminated by a portion of light 14 emitted by laser 46 . This light causes a photocurrent proportional to the light intensity to flow through a photodiode load circuit consisting of resistors 60 and 62 connected in series. The magnitude of this current is digitally encoded by resistors 60, 62 that cooperate with first 64 and second 66 comparators.

By appropriately selecting the inverting and non-inverting inputs and outputs for comparators 64, 66, and by appropriately selecting the resistance values of resistors 60, 62, 68, and 70, the configuration of FIG. The logical signs of the outputs 72, 74 of the comparators 64, 66 are formed to correspond to the sign selection. For example, if the laser output power is
When lower than P ₁ , the photocurrent flowing through the load circuit of photodiode 48 is such that the logic value at points 72 and 74 is "01". When the laser output power is between P ₁ and P ₂ , the logical value of points 72 and 74 is "11". When the laser output power exceeds P ₂ , the logical value of points 72 and 74 is "10".

A logic value of ``0'' on enable line 76 causes the output of AND gates 78, 80 to be ``0'', thus causing point 7
The logic value of 2,74 is the flip-flop 82,8
4, thereby opening the feedback loop. The logic values at points 72 and 74 are transmitted via signal lines 86 and 88 to the digital system 32 or microprocessor, etc.
Available to external systems. The logic value generated externally is sent to the flip-flop via signal lines 90 and 92.
By applying it to the inputs of flip-flops 82 and 84, the logical values at points 72 and 74 can be applied to the flip-flops instead. When the logic value of enable line 76 is "1", AND gates 78 and 80 are connected to point 7.
2,74 logical values are flip-flopped 82,8
Add to input 4. This is a common method of ORing logical signals in ECL. In other logic families, OR gates may be required.

Assuming enable line 76 is ``1'', the next notch clock signal applied from clock 94 to the clock inputs of flip-flops 82, 84
A pulse loads the logic value at points 72, 74 into flip-flops 82,84. Therefore, the outputs 96, 98 of flip-flops 82, 84 are at point 7.
It takes the same logical value as 2,74.

Up/down counter 42 (of FIG. 2) is implemented by cascading two hexadecimal counters 100, 102. counter 1
00 and 102 each give a 4-bit parallel output. The overflow "carry out" terminal CO of counter 102 is connected to the "carry in" terminal CI of counter 100. The two counters 100, 102 thus function as a single counter with an 8-bit counting range and an 8-bit parallel output.

Counters 100, 102 each have the following logic sign dominant operation. When the mode input is "11", the counter holds the count value without changing. When the mode input is ``01'', the counter increments by one when a clock pulse is received at clock input C from clock 94. When the mode input is ``10'', the counter decrements by one when a clock pulse is received at clock input C. When the mode input is "00", the counter loads the count value on the preset terminal. All of the preset terminals are connected to a logic level "0" potential as shown. The presence of pull-up resistors 97 and 99 automatically generates a "00" sign at points 96 and 98 upon initial turn-off. Therefore, the counter 100,
102 always starts with a count value of "00000000" when turned on. This protects the laser from transients that could damage the laser 46 during turn-on.

An overflow of the counter 100, indicated by the presence of a ``1'' at the carry out terminal CO, indicates that a light output within the ``11'' power range cannot be achieved by the feedback controller. This generally means that laser 46 has failed. Therefore, the counter 10 on the signal line 104
An overflow count from 0 is interpreted as a "laser failure flag." If all three of the highest output bit lines 110, 111, and 112 have a logic value of 1 at the same time, this indicates that 7/8 of the range of the control device has been utilized, indicating a laser failure. is interpreted as being close to each other. This condition is detected by AND gate 118, whose "1" output is interpreted as a "laser failure approaching flag."

Output bit line 110 of counters 100, 102
-117 are connected to AND gates 120-127, respectively. The other input of each of these AND gates is connected to enable line 128. AND gates 120-127 are "1" on enable line 128
When enabled by counter 100,
The count value of 102 is transferred via an AND gate and applied to a binary resistor ladder circuit consisting of resistors 130-137. The resistance values are increasing in value in a binary manner as shown such that the sum of the currents flowing through the resistances in the binary ladder circuit is a discrete analog value corresponding to the count value. The binary ladder circuit acts as a digital-to-analog converter or digitally programmable current source. The sum of the currents flowing through the resistors of the binary ladder circuit is added to the laser 46 via line 138 as a drive current. Other D/A low resistance networks such as R2R networks can also be used.

As is well known, pull-up resistors must be used with ECL techniques, but many of the pull-up resistors are not shown to simplify the diagram.
One result of using ECL technology is that the current never reaches a zero value from beginning to end. Therefore, even if the count value is "00000000", line 13
Some current flows through 8. The D/A resistor network should be designed such that this current is not sufficient for laser 46 to reach threshold.

Diodes 140, 142 provide a logical supply voltage (V _EE =5.2 volts) such that the additional voltage drop across laser diode 46 provides the correct termination voltage (V _TT =2 volts) for the resistors in the D/A network. Provided for offset.

When the digital control device is used in optical communication applications, the internal operation sequence of the control device is as follows. When power is turned on, the counter loads a preset zero. Then immediately A/
A D converter encodes the laser output power as insufficient ("01"). This causes the counter to increment, thereby increasing the bias current provided to the laser by the D/A converter via line 138 at each count. This continues until the laser output power falls within the allowable power range ("11") and is encoded. At this time, a modulation signal may be added to the bias current at node 144. The signal input on line 146 is connected to gate 148 and resistor 15.
0 to this node. This modulation current changes the average power produced by the laser. If the average power displacement is sufficient to take it outside the allowable power range ('11'), this is encoded by the A/D converter and the counter is incremented or Decrement the count. In fact, the allowable power range ("11") is chosen to be slightly wider than the average power variation produced by the input modulation signal. The resolution of the programmable current source (D/A converter) is sufficiently large that there is at least one counter state such that variations in the average power introduced by the modulating current do not cause a change in the counter state. The counter enters this state (or one of these states) after a short period of operation. Once this state is achieved, the only phenomena that change the state of the counter are changes in the average power of the laser due to aging or temperature.

When the controller is used in peak power control modulation mode, which may be required in printing, recording or display applications, the sequence of internal operations of the controller is as follows. That is, a logic level zero signal is applied to enable line 76 and external control word line 9.
0,92. Power is then applied to the controller and the counter loads the preset zero. At this time, a digital signal train to be expressed as a peak power controlled optical pulse is applied to enable line 76. As soon as the first logic level 1 is applied to this line, the control loop closes,
The state of the laser is encoded and the counter is incremented until the laser output reaches an acceptable power range ("11"). Acceptable power range is resistance 60,
By appropriately defining 62, it is set to what is required for the application. As long as external control word enable 76 is held at logic level 1, the controller maintains the laser output within an acceptable power range by the mechanism described above. When enable 76 becomes a logic level 0, the zero preset is again loaded into the counter and drops the laser output to zero.
This cycle repeats each time a pulse is applied to enable line 76. A similar modulation of the laser drive current can also be performed using enable line 128 by setting external control word lines 90 and 92 to ``11'' and setting enable line 76 to 0 after the laser is within an acceptable range. It's okay. This mode of operation requires periodically re-enabling the feedback loop to update the counter.

enable line 76 and external control word line 90,
92 may be brought to a logic level 0 to turn off the laser. This signal is provided by an interlock switch on the cabinet in which the laser is mounted or by a signal from the system in which the laser is used. This mechanism provides a means of protecting things from inadvertent exposure to laser radiation. Enable wire 12 to achieve the same function
8 may be used instead.

The controller can test the condition of the laser diode to which it is connected by applying a logic level 0 to enable line 76 and external control word line 92 and a logic level 1 to external control word line 90. can. This causes the counter to continuously increment and the D/A converter to repeatedly scan its dynamic range. The current through the laser diode therefore varies from some value below the threshold to the peak current that the D/A converter can supply. The response of the laser diode to all currents within this range is therefore examined at either the analog laser report line 160 or the laser light output port.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a digital control device for the bias current of a light-emitting semiconductor device according to the invention, FIG. 2 is a block diagram of a preferred embodiment of the digital control device, and FIG. FIG. 4 is a diagram showing the output power curve of a typical injection laser, and FIG. 4 is a diagram of a preferred embodiment of FIG. 10... Light receiving device, 12... Light emitting semiconductor device, 16... Optical fiber, 20... Digital control device, 26... A/D converter, 28...
Control logic, 30...D/A converter, 32...Digital system.

Claims (1)

[Claims] 1. Sensing light output from a light emitting semiconductor device;
a device for generating a digital signal representing the amplitude thereof; control logic for generating a digital output signal in response to said digital signal; and a device for converting said digital output signal into a drive current for a light emitting semiconductor device. Control circuit for semiconductor devices. 2. The control circuit of claim 1, wherein the device for sensing light output comprises a photodiode. 3. The control circuit of claim 1, wherein said control logic comprises an up/down counter, and said digital output signal is a count of said counter.